1. Field of the Invention
The present invention relates to a spin-injection magnetic random access memory in which magnetization reversal is performed with spin-polarized electrons.
2. Description of the Related Art
In addition to magnetic heads and detection elements such as a magnetic sensor, a magnetoresistive element in which a magnetic film is used has been studied for the use of a storage element in a magnetic random access memory (MRAM), which is of a solid-state magnetic memory.
The magnetoresistive element has a sandwich structure, which includes, e.g., two magnetic metal layers and a dielectric layer arranged there between. In the magnetoresistive element having the sandwich structure, because magnetization states of the two magnetic metal layers differ depending on the data, the data can be read by utilizing tunneling magnetoresistance (TMR) effect.
Recently, for an MR ratio (magnetoresistive ratio), which indicates a magnetoresistive fluctuation rate, the magnetoresistive element having the MR ratios more than 20% at room temperature is obtained, and research and development of the magnetic random access memory is actively conducted.
The magnetoresistive element in which the TMR effect is utilized can be realized as follows: After an Al (aluminum) layer having thicknesses ranging from 0.6 nm to 2.0 nm is formed on a ferromagnetic body which is of the magnetic metal layer, a surface of the Al layer is exposed to oxygen glow discharge or oxygen gas to form a tunnel barrier layer made of Al2O3, and the ferromagnetic body which is of the magnetic metal layer is further formed.
Instead of Al2O3, MgO (magnesium oxide) can also be used as the tunnel barrier layer.
A ferromagnetic single-tunnel junction element is proposed as another structure of the magnetoresistive element in which the TMR effect is utilized. In the ferromagnetic single-tunnel junction element, for example, one of two ferromagnetic layers is formed in a magnetic fixed layer whose magnetization state is fixed by an anti-ferromagnetic layer. Further, a magnetoresistive element, which has a ferromagnetic tunnel junction through magnetic particles dispersed in a dielectric body and a ferromagnetic double-tunnel junction element in which the ferromagnetic body is formed in a continuous film are also proposed.
These magnetoresistive elements have been considered to have a high potential for application, because the MR ratios range from 20% to 230% and a decrease in MR ratio is suppressed even if voltage applied to the magnetoresistive element is increased.
In the magnetic random access memory in which the magnetoresistive element is used, a readout time is as fast as not more than 10 nano seconds and rewritable endurance is as large as at least 1015 times.
However, the data writing (magnetization reversal) in the magnetic recording layer is performed with a magnetic field generated by pulse current. Therefore, current density of the pulse current supplied to a word line or a bit line is increased, which results in new problems that electric power consumption is increased, large memory capacity is difficult to be realized, and a driver for generating the pulse current is increased in an area.
Therefore, there is proposed a yoke wiring technology in which a magnetic material (yoke material) having high magnetic permeability is provided around a writing line to impart efficiently the magnetic field to the magnetoresistive element. According to this technology, the current density of the pulse current generated during the data writing can be decreased.
However, the pulse current cannot still be decreased to a value required for the practical use of the magnetic random access memory, i.e., the values not more than 1 mA.
The writing method by the spin injection is proposed as the technology, which solves these problems at a stroke.
The spin-injection writing method has a feature in that the magnetization reversal of the magnetic recording layer is performed by injecting the spin-polarized electrons into the magnetic recording layer of the magnetoresistive element.
When the magnetization reversal is performed by the spin-polarized electrons, because the current density of the pulse current can be decreased compared with the case in which the magnetization reversal is performed by the magnetic field, the spin-injection writing method can contribute to the electric power consumption reduction, the enlargement of memory capacity, the driver area reduction, and the like. In this case, in order not to generate the ring magnetic field by the pulse current, because it is necessary to decrease dimensions of the magnetoresistive element, it is convenient to integrate the magnetoresistive element.
In order to realize the spin-injection writing method, firstly, it is necessary that thermal stability (thermal fluctuation resistance) is ensured when the dimensions of the magnetoresistive element is equal to or smaller than 0.1×0.1 μm2. Second it is necessary that a fluctuation in dimension of the magnetoresistive element is decreased. Third it is necessary that the current density of the pulse current required for the spin-injection magnetization reversal is decreased.
Currently the current density of the pulse current required for the spin-injection magnetization reversal is about 107 A/cm2, however, the further reduction of the current density is desired in order to prevent a tunnel barrier breakage problem and the like.
In the magnetoresistive element in which giant magnetoresistance (GMR) effect is utilized, the current density of the pulse current can be decreased to a degree of 106 A/cm2 by adopting the so-called dual-pin structure. For example, in the case where Cu/Co90Fe10, Ru/Co90Fe10 is used as a spin reflection film, the current densities of the pulse current required for the spin-injection magnetization reversal become about 8×106 A/cm2 and about 2×106 A/cm2 respectively.
However, these values are not still enough to realize the magnetic random access memory. In order to solve the problems such as the tunnel barrier breakage and the thermal disturbance caused by the temperature rise of the magnetoresistive element, it is necessary to conduct the research and development of a new architecture and writing method, which can realize the further reduction of the current density.